Comparison of actively developed Common Lisp implementations

All the implementations below provide an FFI and sockets interface. Callbacks mean in this context that the FFI is capable of passing lisp-functions as callbacks to foreign functions. The startup file is loaded when the Lisp starts. Not all platforms listed may be actively supported.

Note: the first line below article title is misleading, it advertises 'free', but *features* link shows all including the commercial ones, and this article title does not include 'free'

Note: Some implementation builds fail to include FFI, sockets, and/or threads. Fink CLISP fails to includes threads, and MacPorts anything fails to include FFI. Aptitude lisps are severely out of date: SBCL fails on Xen, and ECL crashes during installation.